I'm really not so sure. People don't really have any reason to pledge allegiance to any particular pool long term, nor is there the real life problem of making a choice (voting) and then being stuck with it for years.
I mine with BTCGuild because I like their website. A day from now I could decide I like someone else and mine in their pool. A day later I could switch back. There is hardly any cost in doing so, for me or any other miner, so any power a pool operator has is extremely temporary and entirely contingent on keeping their miners happy. That makes a pool operator doing bad things seem at least fairly unlikely.
Wrong.
Its a fundamental issue in trust...investor confidence. Once the trust is broken, it won't be restored.
Sooo that means, when a broken chain is discovered (and when can lag), how does the joe-investor who just dropped $100k on btc recover?
He sells...
Which is also a powerful incentive for pool operators to NOT mess with the block chain. If they do it an are discovered, they screw themselves out of all the BTC they're earned so far (which in the case of deepbit is a LOT). If they aren't discovered but there is even the hint of wrongdoing, miners jump ship. The best choice for them is being above board, I think.
But I'm also playing devil's advocate here, to be honest. I would feel more confident about the whole thing if hashing power was distributed a bit more evenly.